error[E0723]: mutable references in const fn are unstable
  --> $DIR/feature-gate-const_mut_refs.rs:5:14
   |
LL | const fn foo(x: &mut i32) -> i32 {
   |              ^
   |
   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
   = help: add `#![feature(const_fn)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0723`.